To reduce the volume of incineration ash resulting from incineration of municipal refuse or the like and to make heavy metals contained in the ash insoluble or harmless, melt treatment of incineration ash, that is, a treatment in which incineration ash is melted at high temperature in a melting furnace and discharged from the melting furnace as molten slag, is carried out. In such a melting furnace, it is necessary, on the one hand, to raise the temperature in the melting furnace to a relatively high temperature, e.g. in the vicinity of 1,300.degree. C., in order to melt incineration ash, and on the other, it is important to control the temperature so as not to exceed 1,300.degree. C. with a view to minimizing damage to the furnace wall. Therefore, it is necessary to detect the temperature in the furnace continuously.
To measure the temperature in the melting furnace, a thermocouple has heretofore been used. However, the thermocouple detects the temperature of the gas in the furnace because it cannot directly measure the temperature of molten slag, which is at high temperature. The melt condition of molten slag in the furnace has heretofore been estimated from the temperature of the thermocouple and the condition of the molten slag, which is visually observed through a peep hole provided in the wall of the melting furnace. However, visual observation made from the access hole of the melting furnace through quartz glass involves problems. For example, because the size and shape of the access hole are limited, the visual field is narrow, and smoke in the furnace can adhere to the quartz glass. Therefore, the visual observation cannot accurately be performed.